Petroleum products obtained by distilling crude oil under atmospheric or vacuum conditions, for example, gas oil, atmospheric residue, vacuum gas oil and vacuum residue frequently contain a number of sulfur compounds the content of which varies depending on the type of the crude oil.
When these hydrocarbon oil components are subjected to combustion, harmful substances such as sulfur compounds are generated. As a result, a combustion chamber is corroded and, further, the atmosphere is contaminated.
In high-ordered processing of the above-mentioned petroleum products such as lightening or improvement in qualities, sulfur compounds, nitrogen compounds and metals contained in these petroleum products would inactivate catalysts and thus seriously disturb the effective utilization of heavy hydrocarbon oils.
Therefore, attempts have been made to eliminate sulfur and metals by contacting hydrocarbon oils containing large amounts of sulfur compounds and metals with catalysts, wherein metals of the group VIA in the periodic table (for example, Mo, Cr, W) or metals of the group VIII metals (for example, Fe, Ni, Co, Pt) are carried on alumina or an alumina-containing carrier, under an elevated hydrogen pressure.
Although such a catalyst can fully eliminate sulfur compounds, metals cannot be fully eliminated thereby in some cases.
In hydrofining atmospheric residue, vacuum residue or vacuum distillate, it is considered to be effective to use a plural number of catalysts differing in pore distribution from each other, as JP-B-49-18763 points out (the term "JP-B" as used herein means an "examined Japanese patent publication").
Specifically, this technique comprises, in a reactor of the fixed bed type, locating a catalyst with a large metal tolerance and having a large pore size in the inlet side followed by a catalyst with a moderate desulfurizing activity and having a moderate pore size, and then locating a catalyst with a high desulfurizing performance and having a small pore size in the outlet side.
This technique is based on an idea that metals contained in the oil to be treated are eliminated in the inlet side of the reactor and the catalyst with a high desulfurizing activity, which is liable to undergo plugging due to metals, in the outlet side is thus protected.
However, fully satisfactory results cannot be achieved even by this technique. Reasons therefor are seemingly as follows.
That is to say, it is thought that a catalyst having a large pore size has a high ability to accumulate metals as the result of the large pore size. However, a catalyst lacking of a high ability to eliminate metals from an oil to be treated cannot be regarded as having an excellent performance, though it has a high ability to accumulate metals.
While taking these points into consideration, the present invention aims at providing a catalyst for hydrofining hydrocarbon oils which has a high ability to eliminate metals contained in the hydrocarbon oils therefrom and a larger metal tolerance as well as a carrier suitable for the preparation of said catalyst.